I'd like to make myself a t-shirt with custom print. Anyone knows if there's a company that can do it?

There are dozens that I've seen; however, the only one that I know the precise location is in Lamai. Right here: http://goo.gl/maps/EOkZz

As JB states - dozens.

Quite a few between Tesco Lotus Chaweng and the lights on the corner of the road to Lam Din market. (Lights not working at the moment.) They are on both sides of the road. One is very close to the lights on the corner by the French Bakery

The one that I buy from sells decent quality and is based a bit further towards Bophut.

Coming from Tesco Lotus Chaweng, head to Bophut. After Surat Baan Suaay and also on the left - just before 'Take a Bread'. (Opposite the new big massage place.)

Thanks, will look for them!

Do they deal low quantities (I need only 1-2), or hundreds?

Dunno about Samui but I get shirts printed for our Hash House Harriers here in Isaan.

Yes local printers will do short runs but they will charge a fixed cost to make up template screens, which can then be used to print anything from 1 to hundreds of shirts.

Here in Isaan the actual printing costs for shirts are low once you have the plates made up. 40 baht to print a front or back face and 20 baht for a sleeve decal - a bit more if you want more than two colours printed. Template screens vary in cost depending on design - 250 baht for a relatively straightforward front or back screen and 100 for a sleeve decal. I'm guessing there will be the island premium though!

So you could well end up paying up to 1,000 baht for the first shirt and only 100 baht for each subsequent one if its a multi-sided plus sleeves print. For a basic print on only front or back the first shirt printing may cost 250 baht and subsequent ones less than 50 baht.

I'm assuming printing on basic cotton T. Plain Ts are surprisingly difficult to get hold of unless white (Big C often has some good quality whites in men and women styles). I have to buy mine in Bangkok Pratunam for the full range and for colours. Ask around the printers - my local one used to have a supplier when I started using him but she moved away.

Two places that I passed yesterday.

The first one is just out of Maenam - almost opposite Napasai (just after the petrol station)

The next one is opposite Relax Spa - near Take a Bread. In between Bophut and Big C

OP may want to consider having a machine embroidered shirt. The fella who programs the machine in Tesco Lotus Lamai knows what he is doing. I take what I want doing on a flash drive and he just gets on with it. Single good quality polo shirt with small motif on the front and whatever on the back for 650 Baht. His missus handles most of the customer stuff and is very go at it. Very professional outfit and a pleasure to do business with.